The strength of glass depends on the intactness of the glass surface. Immediately after glass is produced, its strength is highest. However, the strength is reduced when the glass surface has nicks or scratches. Numerous methods are therefore known for protecting the surfaces of objects of glass against mechanical damage.
For example, it is known to treat glass objects immediately after they are shaped at temperatures between 370.degree. and 750.degree. C. with inorganic or organic compounds of titanium, tin or zirconium (the so-called hot-end finishing). With this treatment, thin, colorless, transparent, protective metal oxide layers are formed on the surfaces of the hot-finished glass objects.
To further increase the scratch hardness, the strength and especially the smoothness of glass objects, it is known that an additional protective organic layer may be applied on glass objects which are coated with metal oxides.
From the extensive patent literature, the following are cited as being illustrative of the state of the art.
In the German Pat. No. 1,291,448, a method is described to increase the scratch hardness and strength of glass objects, especially of glass bottles, by producing a thin, colorless, transparent, protective layer on the external surfaces of the glass objects. The characteristic feature of this method resides in that thin layers of a pyrolyzable inorganic salt or of a pyrolyzable organic compound of titanium, zirconium, tin or vanadium, applied on the glass objects, are decomposed pyrolytically on the glass objects at temperatures between 370.degree. and 705.degree. C. to the corresponding metal oxides, whereupon the glass objects are cooled to temperatures between 230.degree. and 65.5.degree. C. and an olefin polymer, a polyurethane, a polystyrene or an acetate salt of an alkylamine are sprayed on the still hot glass surfaces. An example of an olefin polymer is a polyethylene wax of low molecular weight, which is used in the form of an aqueous emulsion. As emulsifier, the alkali metal salt of a fatty acid, especially potassium oleate, is used. The thickness of the protective layer on the glass object is about 1 .mu.m. These films of polyethylene wax are, however, not resistant to soap suds and, when the container is cleaned, are detached after a relatively short time by the wash-active substances of the hot cleaning liquor.
German Pat. No. 1,298,681 discloses a method for coating glass objects with a wear-resistant and slideable coating. This method is characterized in that an aqueous solution of a reaction product of polyvinyl alcohol, an emulsified polyolefin, preferably emulsified polyethylene, and an acid compound is applied to the surface of the glass object.
The German Pat. No. 1,596,742 relates to a method for preparing long-lasting smooth protective layers on glass objects, for which the protective layer is applied from a mixture of a dispersed polyolefin and a polyoxyethylene derivative of a fatty acid on the glass object heated to 70.degree. to 225.degree. C. The disperse polyolefin preferably is polyethylene. Preferably polyoxyethylene monostearate is used as polyoxyethylene derivative of a fatty acid.
The German Pat. No. 2,432,478 discloses a glass container with a protective layer of an ionic copolymer of an .alpha.-olefin and an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, the glass container additionally having a sliding coating with the following components in parts by weight:
1 part of at least one of the following salts: calcium stearate, zinc stearate, calcium oleate, zinc oleate; PA1 0.5 to 2 parts of a soluble polyvinyl alcohol; and PA1 1.7 to 3.6 parts of at least one of the following oleates: potassium oleate, sodium oleate and ammonium oleate.
The coating materials described in the prior art, are, however, not able to fulfill all the requirements for a satisfactory protective layer material for glass surfaces. These protective layers must fulfill especially the following demands:
The protective layers should effectively protect the glass surface against mechanical damage, such as occurs when the glass containers rub against one another during cleaning, filling or packaging or come into frictional contact with metal surfaces of the cleaning, filling or packaging machines.
The protective layers should be able to withstand numerous cleaning and washing procedures, for example, in dishwashing machines. The protective layers must also be resistant to attack by hot, aqueous washing liquors at elevated temperatures.
The protective layers must have a good sliding capability so that the treated glass containers are readily manageable. The term "sliding capability" means that the layers act in the manner of a lubricant when contacted by another object such as another glass bottle.
The protective layers should adhere well to glass surfaces, so that, when damaged at a point or a small area, they cannot readily be pulled from the glass surface.
Labels, especially labels with a pressure-sensitive coating, must adhere adequately to the protective layers of the coating to ensure that the treated glass containers can be labelled.
The protective layers must be physiologically safe, since the majority of glass containers having such protective layers are used for packaging foods or beverages.
Finally, the protective layers should not significantly increase the ultimate price of the glassware so treated.